Field of the Invention
The present invention relates to a contactless power transmission device and a power transfer system, and particularly to a power control technique in a contactless power transmission device that transmits electric power to a power reception device in a contactless manner.
Description of the Background Art
Japanese Patent Laying-Open No. 2014-207795 discloses a contactless power feeding system that supplies electric power from a power feeding device (power transmission device) to a vehicle (power reception device) in a contactless manner. In this contactless power feeding system, the power feeding device includes a power transmission coil, an inverter and a control unit. The power transmission coil transmits electric power to the power reception coil mounted on the vehicle in a contactless manner. The inverter produces an AC current in accordance with a drive frequency for output to the power transmission coil. The control unit obtains a charging power command for a battery and output power for the battery from the vehicle side, and controls by feedback the drive frequency of the inverter such that the output power follows the charging power command (see Japanese Patent Laying-Open No. 2014-207795).
When the inverter is a voltage-source inverter and supplies transmission power in accordance with the drive frequency to the power transmission unit, transmission power can be controlled by adjusting the duty of an inverter output voltage. By adjusting the drive frequency of the inverter, a turn-on current indicating an inverter output current at the rising of the inverter output voltage can be controlled.
It is known that in the voltage-source inverter, if an output current of the same sign as the output voltage (i.e., positive turn-on current) flows at the rising of the output voltage, a recovery current flows into freewheel diodes of the inverter. When a recovery current flows, the freewheel diodes generate heat, resulting in increase in losses. Therefore, by adjusting the drive frequency of the inverter to control the turn-on current to a target value (e.g., less than or equal to 0), the recovery current can be prevented from occurring.
However, in the course of adjusting the drive frequency of the inverter to search for a desired inverter operating point, a current supplied from the inverter to the power transmission unit may exceed a predetermined limit value (rated current, etc.) to cause an overcurrent. The above-described Japanese Patent Laying-Open No. 2014-207795 does not particularly discuss the subject of suppressing such an overcurrent and a solution thereto.